Hey y'all! (new to the forums) but have been lurking & decided it was time to join!
Anyways I have a few theories on Bing Bong. I'm drawn to believe he's Riley's childhood imaginary friend, who, as she's thrown into emotional turmoil and is growing up, is starting to fade from importance - perhaps even cast into the furthest reaches of the mind. One account I read on the internet said that elements in the Imagination Land are changing as Riley grows...one attraction turns into a "Dream Boyfriend Generator" etc., so perhaps as Riley's world expands, imaginary friends have fewer places to linger...And on Joy & Sadness' journey across the mind, they may have to rescue him before it's "too late". Or maybe he helps them save Headquarters.
Notice in those decals, he's seen carrying Joy & Sadness in the upper left sticker.
Also a toy was announced, that "smells like cotton candy" and I saw some description (I forget where) saying he's a Cotton Candy Elephant who cries candy. Lending to the imaginary friend belief. Also, costume wise...the gloves & jacket just make me think of a drifter of sorts, who would be found riding around the train of thought.
Just some of my thoughts from various things I've seen across the internet Super excited. 4 months!!